Familial hypertrophic cardiomyopathy (FHC) is a disease characterized by cardiac hypertrophy, myofibrillar disarray and sudden death. FHC results from autosomal dominant mutations in sarcomeric proteins. The goal of this proposal is to provide a molecular basis for FHC in patients with mutations in the myosin regulatory light chain (RLC). These studies are a necessary precursor to development of therapeutic protocols that will attenuate the effects of heart failure. The mutations are localized near the phosphorylatable serine, the EF hand, and the MHC binding regions of the RLC molecule and, since the RLC binding region of myosin is thought to undergo large conformational changes that drive muscle contraction, we will address fundamental aspects of RLC function and the molecular basis of myosin motion generation. Our approach will utilize laser-trapbased in vitro force and motility assays to assess the effects of RLC mutations on: 1) unloaded shortening velocity, 2) isometric force, 3) Ca++ sensitivity and 4) power production from mutant myosin ensembles. Any alterations in ensemble behavior will be further pursued at the single myosin molecule level to determine if they result from altered mechanical properties or from changes in the rates ofactomyosin kinetic transitions. Our approach measures the mechanical properties of isolated contractile proteins. Since the method is free from the ambiguities arising from mechanical experiments using skinned fiber and whole heart preparations (e. g.: myocyte disarray, fibrosis, and diffusion limitations), it is the purest way to determine the direct effects of the FHC mutations on actomyosin force and power generation. Knowledge of how the RLC mutations affect myosin's inherent function will allow the degree of alteration to higher functional units, such as the cardiac muscle fiber, or the heart itself to be correlated with a primary contractile defect. [unreadable] [unreadable] [unreadable]